[services] Tunnelbroker - Changing Buildkite commands for Tunnelbroker to run cargo

Summary:
This diff introduces changes to the Buildkite to change the build commands for Tunnelbroker from cmake to use cargo build and cargo test instead. We are removing the target directory before building to remove the cache.
Also, the building and running of the tests for the legacy C++ code are added.

Related Linear task: ENG-2062

Test Plan:
To test manually, we can run the Buildkite commands in the root of the repo to ensure they are successful.

Running the following commands in a repo root directory will be expected to succeed:

cd services/tunnelbroker
rm -rf target
nix develop --accept-flake-config -c bash -c "cargo build && cargo test"
cd src/libcpp/test
rm -rf build
nix develop --accept-flake-config -c bash -c "cmake -B build . && make -C build -j"
build/bin/runTests --gtest_filter="-AmqpManager*:DatabaseManager*"

Reviewers: jon, varun, marcin, tomek

Reviewed By: jon, tomek

Subscribers: ashoat, tomek, atul, abosh

Differential Revision: https://phab.comm.dev/D5461